In general, a heat exchanger is a device that passively transfers heat from one material to another. The materials are typically either liquid or gaseous. The work done by heat exchangers is based on the laws of thermodynamics in that when an object is heated, the heat energy contained within that object will diffuse outward to the surrounding environment until equilibrium has been reached between the object and the environment. Heat exchangers have a wide variety of uses including, but not limited to, space heating, refrigeration, air conditioning, heat transfer in power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment.
The basic heat exchanger design generally involves two chambers or passageways separated by a barrier. A feed stream with thermal energy to be transferred is passed through one passageway, and a stream that may receive thermal energy is passed through the other. The streams may be in either co- or counter-current flow. Heat diffuses between the two streams by passing through the barrier until the rate of heat transfer is matched by the resistance to such transfer. The type and size of heat exchanger used depends on the type of material used as the barrier and various properties of the material streams introduced including their phase, temperature, density, viscosity, pressures, chemical compositions, and various other characteristics.
The two most common types of heat exchangers are shell and tube heat exchangers, and plate and frame heat exchangers. Other types of heat exchangers include, but are not limited to, adiabatic wheel heat exchangers, plate fin heat exchangers, fluid heat exchangers, waste heat recovery units, dynamic scraped surface heat exchangers, phase-change heat exchangers, direct contact heat exchangers, air coils, and spiral heat exchangers.
A spiral heat exchanger may refer to a helical tube configuration, but more often is a pair of flat surfaces that are coiled to form two channels in a counter-flow arrangement. Spiral heat exchangers are generally associated with efficient use of space and low energy costs. Some applications include, but are not limited to, pasteurization, digester heating, heat recovery, pre-heating, effluent cooling, and sludge treatment.
Shell and tube heat exchangers consist of a series of tubes arranged in bundles and suspended within shells by insertion into spacer plates. They are considered robust because of the strength of their materials, and effective because of their materials' high heat coefficients and the fluid mixing profiles caused by their design. They are typically suited for higher-pressure and higher temperature applications and often used in oil refineries and other large chemical processes. Tubes must be made of a strong, thermally conductive, corrosion-resistant material. High quality tube materials are typically metals, including, but not limited to, copper alloy, stainless steel, carbon steel, non-ferrous copper alloy, Inconel, nickel, Hastelloy, and titanium.
A plate heat exchanger is composed of multiple, thin, slightly separated plates that have large surface areas and fluid flow passages for heat transfer. The plates are usually made of metal. Plate heat exchangers are generally compact and associated with a high overall heat transfer coefficient. Fluids spread out over the plates thus facilitating the transfer of heat and greatly increasing the speed of temperature changes. They are well suited to transfer heat between medium- and low-pressure materials. Large heat exchangers of this type are called plate-and-frame and are often used in heating, ventilation, and air-conditioning applications. Small plate heat exchangers are used in domestic heating and hot water. Very small brazed versions are used in hot-water sections of combination boilers.